On topological properties of the Hartman Mycielski functor

Proc. Indian Acad. Sci. (Math. Sci.) Vol. 115, No. 4, November 2005, pp. 477 482. Printed in India On topological properties of the Hartman Mycielski functor TARAS RADUL and DUŠAN REPOVŠ Departmento de Matematicas, Facultad de Cs. Fisicas y Matematicas, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile Institute for Mathematics, Physics and Mechanics, University of Ljubljana, Jadranska 19, Ljubljana, Slovenia 1001 E-mail: tarasradul@yahoo.co.uk; dusan.repovs@fmf.uni-lj.si MS received 2 January 2005 Abstract. We investigate some topological properties of a normal functor H introduced earlier by Radul which is some functorial compactification of the Hartman Mycielski construction HM. We prove that the pair (HX,HMY ) is homeomorphic to the pair (Q, σ ) for each nondegenerated metrizable compactum X and each dense σ -compact subset Y. Keywords. Hilbert cube; Hartman Mycielski construction; equiconnected space; normal functor; metrizable compactum; absolute retract. 1. Introduction The general theory of functors acting on the category Comp of compact Hausdorff spaces (compacta) and continuous mappings was founded by Shchepin [Sh]. He described some elementary properties of such functors and defined the notion of the normal functor which has become very fruitful. The classes of all normal and weakly normal functors include many classical constructions: the hyperspace exp, the space of probability measures P, the superextension λ, the space of hyperspaces of inclusion G, and many other functors (see [FZ] and [TZ]). Let X be a space and d an admissible metric on X bounded by 1. By HMX we shall denote the space of all maps from [0, 1) to the space X such that f [t i,t i+1 ) const, for some 0 = t 0 t n = 1, with respect to the following metric: d HM (f, g) = 1 0 d(f (t), g(t))dt, f,g HMX. The construction of HMX is known as the Hartman Mycielski construction [HM]. Recall, that the Hilbert cube is denoted by Q, and the following subspace of Q {(a n ) n=1 Q a k = 0 for all but finitely many k} is denoted by σ. Telejko has shown in [Te] that for any nondegenerated separable metrizable σ -compact strongly countable-dimensional space X the space HMX is homeomorphic to σ. For every Z Comp consider HM n Z = { f HMZ there exist 0 = t 1 < <t n+1 = 1 with f [t i,t i+1 ) z i Z, i = 1,...,n }. 477

478 Taras Radul and Dušan Repovš Let U be the unique uniformity of Z. For every U U and ε>0, let α, U, ε ={β HM n Z m{t [0, 1) (α(t), β(t )) / U} <ε}. The sets α, U, ε form a base of a compact Hausdorff topology in HM n Z.Givenamap f : X Y in Comp, define a map HM n X HM n Y by the formula HM n F(α) = f α. Then HM n is a normal functor in Comp (see [TZ], 2.5.2). For X Comp we consider the space HMX with the topology described above. In general, HMX is not compact. Zarichnyi has asked if there exists a normal functor in Comp which contains all functors HM n as subfunctors [TZ]. Such a functor H was constructed in [Ra]. We investigate some topological properties of the space HX which is some natural compactification of HMX. The main results of this paper are as follows: Theorem 1.1. HXis homeomorphic to the Hilbert cube for each nondegenerated metrizable compactum X. Theorem 1.2. The pair (H X, HMY) is homeomorphic to the pair (Q, σ ) for each nondegenerated metrizable compactum X and each dense σ -compact strongly countabledimensional subset Y. 2. Construction of H Let X Comp. ByC(X) we denote the Banach space of all continuous functions ϕ : X R with the usual sup-norm: ϕ =sup{ ϕ(x) x X}. We denote the segment [0, 1] by I. For X Comp let us define the uniformity of HMX. For each ϕ C(X) and a,b [0, 1] with a<bwe define the function ϕ (a,b) :HMX R by Define ϕ (a,b) = 1 (b a) b a ϕ α(t)dt. S HM (X) ={ϕ (a,b) ϕ C(X) and (a, b) [0, 1)}. For ϕ 1,...,ϕ n S HM (X) define a pseudometric ρ ϕ1,...,ϕ n on HMX by the formula ρ ϕ1,...,ϕ n (f, g) = max{ ϕ i (f ) ϕ i (g) i {1,...,n}}, where f, g HMX. The family of pseudometrics P ={ρ ϕ1,...,ϕ n n N, where ϕ 1,...,ϕ n S HM (X)}, defines a totally bounded uniformity U HMX of HMX [Ra]. For each compactum X we consider the uniform space (H X, U HX ) which is the completion of (HMX, U HMX ) and the topological space HXwith the topology induced by the uniformity U HX. Since U HMX is totally bounded, the space HX is compact. Let f : X Y be a continuous map. Define the map HMf :HMX HMY by the formula HMf(α) = f α, for all α HMX. It was shown in [Ra] that the map HMf : (HMX, U HMX ) (HMY, U HMY ) is uniformly continuous. Hence there exists the continuous map Hf : HX HY such that Hf HMX = HMf. It is easy to see that H : Comp Comp is a covariant functor and HM n is a subfunctor of H for each n N.

3. Preliminaries On topological properties of the Hartman Mycielski functor 479 All spaces are assumed to be metrizable and separable. We begin this section with the investigation of certain structures of equiconnectivity on HXfor some compactum X. The first one will be the usual convexity. Let us remark that the family of functions S HM (X) embed HMX in the product of closed intervals ϕ (a,b) S HM (X) I ϕ (a,b), where I ϕ(a,b) = [min x X ϕ(x), max x X ϕ(x) ]. Thus, the space HX is the closure of the image of HMX. We denote by p ϕ(a,b) : HX I ϕ(a,b) the restriction of the natural projection. Lemma 3.1. HX is a convex subset of ϕ (a,b) S HM (X) I ϕ (a,b). Proof. It is enough to prove that for each α, β HMX ϕ (a,b) S HM (X) I ϕ (a,b) we have that 2 1 α + 2 1 β HX. Let 0 = r 0 <r 1 < <r k = 1 and 0 = p 0 <p 1 < <p m = 1 be the decompositions of the unit interval which correspond to α and β. Consider any ϕ 1,...,ϕ n C(X) and (a i,b i ) (0, 1) for i {1,...,n}. Choose a decomposition of the unit interval 0 = t 0 < t 1 < < t s = 1 such that {r 0,...,r k } {p 0,...,p m } {a i i {1,...,n}} {b i i {1,...,n}} {t 0,...,t s }. Define γ HMX as follows. Let t [t l,t l+1 ) for some l {0,...,s 1}. Define γ(t) = α(t) if t< 2 1 (t l + t l+1 ) and β(t) otherwise. It is easy to see that p ϕ i (γ ) = (ai,b i ) p ϕ i ( 1 (ai,b i ) 2 α + 2 1 β) for each i {1,...,n}. Hence 2 1 α + 2 1 β Cl(HMX) = HX. The lemma is proved. COROLLARY 3.2 HX is absolute retract for each metrizable compactum X. Define the map e 1 :HMX HMX I HMX by the condition that e 1 (α 1,α 2, t)(l) is equal to α 1 (l) if l < t and α 2 (l) in the opposite case for α 1,α 2 HMX, t I and l [0, 1). We consider HMX with the uniformity U HMX and I with the natural metric. Lemma 3.3. The map e 1 : HMX HMX I HMX is uniformly continuous. Proof. Let us consider any U U HMX. We can suppose that U ={(α, β) HMX HMX ϕ (0,1) (α) ϕ (0,1) (β) <δ}, for some δ>0and ϕ C(X). The proof of the general case is the same. Put c = max x X ϕ(x). Choose n N such that 1/n<δ/2cand put a i = i/n for i {0,...,n}. Consider an element V of uniformity U HMX defined as follows: { V = (α, β) HMX HMX ϕ (ai,a i+1 )(α) ϕ (ai,a i+1 )(β) < δ 2n 2, } for each i {0,...,n 1}. Put E ={(l, s) I I l s < 1 2n }.

480 Taras Radul and Dušan Repovš Let us consider any ((α 1,β 1 ), (α 2,β 2 ), (l, s)) V V E. Then we have ai+1 (ϕ α 1 (t) ϕ α 2 (t))dt a i 1 ai+1 = n a i+1 a (ϕ α 1 (t) ϕ α 2 (t))dt i a i = n ϕ (ai,a i+1 )(α 1 ) ϕ (ai,a i+1 )(α 2 ) <n δ 2n 2 = δ 2n, for each i {0,...,n 1}. We have the same for β 1 and β 2. Since t 1 t 2 < 2n 1, there exists i 0 {0,...,n 1} such that t 1,t 2 [a i0,a i0 +1]. Then we have ϕ (0,1) (e 1 (α 1,β 1,t 1 )) ϕ (0,1) (e 1 (α 2,β 2,t 2 )) 1 = (ϕ e 1 (α 1,β 1,t 1 )(t) ϕ e 1 (α 2,β 2,t 2 )(t))dt 0 i 0 1 i=0 ai+1 a i n 1 + i=i 0 ai+1 a i (ϕ α 1 (t) ϕ α 2 (t))dt + c(a i 0 +1 a i0 ) (ϕ β 1 (t) ϕ β 2 (t))dt < n 1 2n δ + δ 2 <δ. Therefore (e 1 (α 1,β 1,t 1 ), e 1 (α 2,β 2,t 2 )) V. Thus the lemma is proved. Hence there exists the extension of e 1 to the continuous map e : HX HX I HX. It is easy to check that e(α, α, t) = α, for each α HX. 4. Proofs Proof of Theorem 1.1. Since HXis infinite-dimensional convex compactum, Theorem 1.1 follows by Klee Theorem [Kl]. We will use the equiconnectivity defined by the function e to prove Theorem 1.2. Let (Z, e) be an equiconnected space where e : Z Z I Z is the map which defines the structure of equiconnectedness. A subset A Z is called e-convex if e(a, b, t) A, for each a, b A and t I. Consider any n N. Let D n 1 be a standard n 1-simplex in R n. Define a map e n : Z n D n 1 Z as follows. Consider any (a 1,...,a n,λ 1,...,λ n ) Z n D n 1. Define the finite sequence {x 1,...,x n } by induction. Put x 1 = a 1. Suppose that we have already defined x j for each j i 1 where 1 <i n. Define ( i 1 l=1 x i = e x i 1,a i, λ ) l il=1. λ l Put e n (a 1,...,a n,λ 1,...,λ n ) = x n. One can check that the function e n is continuous. Let us recall that A Z is homotopically dense in Z if there exists a homotopy H : Z I Z such that H(Z (0, 1]) A.

On topological properties of the Hartman Mycielski functor 481 Lemma 4.1. Let (Z, e) be an equiconnected absolute retract and let A Z be a dense e-convex subset. Then A is homotopically dense in Z. Proof. It is enough to prove that for every n N, every point z Z and every neighborhood U Z of z there exists a neighborhood V of z such that every map f : I n V A admits an extension F : I n U A [To]. Since e n+1 (z,...,z,λ 1,...,λ n+1 ) = z for each (λ 1,...,λ n+1 ) D n, there exists a neighborhood V of z such that e n+1 (z 1,...,z n+1,λ 1,...,λ n+1 ) U for each z 1,...,z n+1 V and (λ 1,...,λ n+1 ) D n. Consider any map f : I n V A. Choose any metric d on I n. Let us consider a Dugunji system for I n \ I n, that is, an indexed family {U s,a s } s S such that (1) U s I n \ I n, a s I n (s S), (2) {U s } s S is a locally finite cover of I n \ I n, (3) If x U s, then d(x,a s ) 2d(x, I n ) for s S (see of Ch. II, 3 of [BP]) for more details. Since dim I n = n, we can suppose that for each x I n \ I n there exists a neighborhood of X which meets at most n + 1 elements of {U s } s S. Moreover we can suppose that the index set is countable. Choose some partition of unity {b s } s S inscribed into {U s } s S. Fix some order S ={s 1,s 2,...}. Define a function F : I n U as follows. Put F(x) = f(x) for each x I n. Now consider any x I n \ I n. There exists a finite sequence (s i1,...,s in+1 ) such that x / U s for each s S \{s i1,...,s in+1 }. Put F(x) = e n+1 (f (a si1 ),...,f(a sin+1 ), b si1 (x),...,b sin+1 (x)). One can check that the function F is a continuous extension of the function f. Since A is e-convex, it follows that f(i n ) A. The lemma is thus proved. Proof of Theorem 1.2. It is easy to check that HMX is an e-convex subset of HX. Since HX is an equiconnected absolute retract, and HMY is a dense e-convex subset, homeomorphic to σ [Te], Theorem 1.2 follows by [BRZ]; Proposition 3.1.7 and Lemma 4.1. Acknowledgement This research was supported by the research grant SLO-UKR 02-03/04. References [BRZ] Banakh T, Radul T and Zarichnyi M, Absorbing sets in infinite-dimensional manifolds (Lvow: VNTL) (1996) [BP] Bessaga C and Pelczynski A, Selected topics in infinite-dimensional topology (Warsaw: PWN) (1975) [FZ] Fedorchuk V V and Zarichnyi M M, Covariant functors in categories of topological spaces, Results of Science and Technology, Algebra. Topology. Geometry, VINITI, Moscow, vol. 28, pp. 47 95 (Russian) [HM] Hartman S and Mycielski J, On the embedding of topological groups into connected topological groups, Colloq. Math. 5 (1958) 167 169 [Kl] Klee V, Some topological properties of convex set, Trans. Am. Math. Soc. 78 (1955) 30 45

482 Taras Radul and Dušan Repovš [Ra] Radul T, A normal functor based on the Hartman Mycielski construction, Mat. Studii 19 (2003) 201 207 [Sh] Shchepin E V, Functors and uncountable powers of compacta, Usp. Mat. Nauk 36 (1981) 3 62 (Russian) [Te] Telejko A, On the Hartman Mycielski construction, preprint [TZ] Telejko A and Zarichnyi M, Categorical topology of compact Hausdorff spaces (Lviv: VNTL) (1999) p. 263 [To] Toruńczyk H, Concerning locally homotopy negligible sets and characterization of l 2 -manifolds, Fund. Math. 101 (1978) 93 110